Concentration detector for colored toner

ABSTRACT

Apparatus for detecting concentrations of colored toner particles in the presence of black or other absorbing toner particles including a light source, a light detector having a field of view of less than about ±10 degrees and preferably less than ±5 degrees, apparatus for supplying a dispersion of colored toner particles optionally containing undesirable contamination by black toner particles between the light source and the detector and computing circuitry operative for determining the concentration of the colored toner particles utilizing output from the light detector.

FIELD OF THE INVENTION

The present invention relates to the field of concentration detectionand more particularly to the detection of toner concentration in colorliquid developer compositions especially in the presence ofcontaminants.

BACKGROUND OF THE INVENTION

In liquid developer systems the liquid developer is generally comprisedof a carrier liquid and toner particles in a generally constant ratio.During imaging operations the concentration of toner particles isreduced and concentrated toner is added to return the concentration toits desired value.

It is important that the concentration of particles should be keptwithin a given range in order to realize consistent copy quality. Thisrequirement is especially important in color printers or copiers, wherethe quality of the images is especially dependent on the color balanceand on its stability.

In general, concentration of toner particles in liquid developers isdetermined by measuring the attenuation of light passing through a givenpath filled with the liquid developer. Since the particles absorb andscatter light, the attenuation of the light is related to theconcentration of the particles.

U.S. Pat. No. 4,579,253 describes a system in which the beam of light issplit into two components only one of which is attenuated by the liquiddeveloper. The concentration is determined from the ratio of theattenuated and unattenuated beams.

Such systems work fairly well in single color systems or in multicolorsystems in which there is no cross contamination between the colors. Ingeneral, the most troublesome cross-contamination is black tonerparticles in a relatively low attenuation color such as yellow. Sinceblack has an attenuation several times that of yellow, visuallynegligible black contamination can effect the determination of the colorconcentration in a way which seriously disturbs the color balance of thesystem.

Japanese Patent Publication Kokai 1-148943 describes a system in whichthe attenuation of beams of light having two different colors aresequentially measured. Using these attenuation values, the publicationdescribes a method for determining the concentration of both the blackand the color particles.

Generally speaking, measurement of attenuation caused by toner particlesis most effective when a large cross-section is viewed, since thisincreases the signal level for the brightness of light source. Thispreferred operation requires a detector with a wide field of view.

SUMMARY OF THE INVENTION

The present invention is based on an analysis of the different factorswhich are operative in the attenuation of light by toner particlesdispersed in a carrier liquid.

The two main factors are the absorption of light and the scatter oflight by the particles. In general, for black toner particles, theeffect of scatter is very small compared to the effect of absorption. Onthe other hand, for colored toner particles, especially for yellow, theeffect of scatter is much greater than that of absorption.

In a preferred embodiment of the invention, the relative sensitivity toscattering measurements is improved by reducing the angle of view of adetector which detects light passing through the dispersion.

When a wide field of view is used much of the light which reaches thedetector may be caused by multiple scattering. When black or othermainly absorbing toners are measured this effect is unimportant.However, when colored toners are present this multiple scatteringgreatly reduces the sensitivity of the system.

For maximum sensitivity, light which is scattered by a colored tonerparticle should be removed from the system and should not reach thedetector. However, when the angle of view of the detector is wide, lightwhich is scattered is generally rescattered and a portion of therescattered light reaches the detector. The larger the angle the greaterthe problem.

In a preferred embodiment of the invention the angle of view of thedetector is limited to less than ±10 degrees, preferably less than ±5degrees. At these angles, the relative sensitivity to color over blackis increased. The only lower limit on the angle is the brightness of thesource, since the total signal received by the detector decreases withdecreasing angle.

There is therefore provided in a preferred embodiment of the inventionapparatus for detecting concentrations of colored toner particles in thepresence of black or other absorbing toner particles including:

a light source;

a light detector having a field of view of less than about ±10 degrees,preferably less than about ±5 degrees;

means for supplying a dispersion of colored toner particles optionallycontaining undesirable contamination by black toner particles betweenthe light source and the detector; and

computing circuitry operative for determining the concentration of thecolored toner particles utilizing output from the light detector.

In a preferred embodiment of the invention the light source includes alamp and a collimating lens. Preferably, the light detector includes afocusing lens.

In a preferred embodiments of the invention the field of view is limitedby the size of the detector, the size of the focusing lens and/or aniris.

Preferably, the lamp is a laser diode or other laser source. Analternative preferred source is a LED or a bright lamp.

In a preferred embodiment of the invention the amount of blackcontamination can be determined. In this embodiment there is furtherprovided a second light detector having a large field of view andcomputing circuitry operative for determining the concentration of theblack toner particles utilizing output from the second light detector.

There is further provided, in accordance with a preferred embodiment ofthe invention a method for detecting the concentration of colored tonerparticles in a dispersion undesirably contaminated by black tonerparticles, the method including the steps of:

providing a quantity of the dispersion of colored toner particlesoptionally contaminated with black toner particles;

illuminating the dispersion;

detecting an amount of light passed through the dispersion with aviewing angle of less than ±10 degrees; and

determining the concentration of the colored toner particles utilizingthe detected amount of light.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following detaileddescription of the preferred embodiments of the invention in conjunctionwith the following drawings in which:

FIG. 1 is a schematic illustration of a concentration detectorconstructed and operative in accordance with an embodiment of thepresent invention; and

FIG. 2 is a schematic illustration of a concentration detectorconstructed and operative in accordance with a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1 which illustrates a concentrationdetector constructed and operative in accordance with a preferredembodiment of the present invention.

The concentration detector typically comprises a source of light 10which may be a lamp or a laser diode, a collimating lens 12 whichcollimates the light from lamp 10 and passes it through a dispersion 14of toner particles in carrier liquid. Preferably, an iris 11 is includedto improve the collimation, i.e., to reduce the angular extent of thelight. Preferably, dispersion 14 flows through channel between lens 12and a focusing lens 16. Focusing lens 16 focuses the light which passesthrough dispersion 14 onto a detector element 18, for example aphotodiode or a photoresistor. The system optionally includes an iris 20at the focus of the light for limiting the field of view seen bydetector element 18. Alternatively or additionally the size of theactive area of detector element 18 will limit the field of view to adesired angle by increasing the distance from lens 16 and detectorelement 18.

In a preferred embodiment of the invention source 10 is a laser diode orLED having a high brightness and a comprising virtually a point sourceis used and iris 11 may be eliminated. A bright point source allows forsubstantial reduction of viewing angle and optimal application of theinvention.

In a preferred embodiment of the invention, detector element 18 is aphotodiode having a very small photosensitive area and iris 20 may notbe required.

In a preferred embodiment of the invention the angle of view of detector18 is limited to less than about ±10 degrees, more preferably to lessthan about ±5 degrees. As will be seen below, the smaller the angle thegreater the selectivity of the system to changes in colored tonerconcentration as compared to black toner contamination. The onlypractical limitation on angle is the strength of the source and thesensitivity of the detector.

In an alternative, especially preferred, embodiment of the invention,lenses 12 and 16 are replaced by planar transparent elements, and acollimator is placed after the laser source 10 so as to irradiate only arelatively limited area of the dispersion 14. In this embodiment, theviewing angle is limited as described above, however, the rejection ofdoubly scattered light is greatly improved. In essence, the requirementthat the light come from a small area of the dispersion and arrive at asmall angle of incidence, greatly enhances the yellow sensitivity.Preferably, iris 20 is included to further reduce the radiation angle ofsource 10, thereby further reducing the scattered light received bydetector 18.

It has been found by the present inventors that as the viewing angle ofdetector 18 is reduced, the sensitivity to colored toner, defined as thepercent change in signal for a given change in color toner concentrationdivided by the nominal signal, increases. The sensitivity to black ismuch less dependent on the viewing angle.

Thus as the angle is decreased, the measurement of the concentration ofcolored toner becomes much less sensitive to contamination by blackparticles than for wide fields of view.

In particular, in a typical wide angle system for measuring yellow tonerparticle concentration, the sensitivity to black contamination is about50 times that to yellow. Thus, even very small and visually unimportantamounts of black toner contamination, causes large errors in themeasurements of yellow concentration. When the angle is limited to about±5 to ±10 degrees the relative sensitivity is reduced to a factor ofabout 3 and 10 respectively, depending on the angle of view. Formeasurements of magenta toner particles the relative sensitivity isreduced from about 30 to about 2-5 depending on the angle.

During calibration one or more dispersions of colored toner particlesare tested and the resulting detector output signals are measured. Basedon these measurements a sensitivity curve for the system and/or thesignal level at which additional toner concentrate should be added tothe system is determined. This value and possibly other points on acalibration curve is stored in computing circuitry 24 which may includea computer such as a microprocessor or dedicated analog and/or digitalcircuitry. In operation in a preferred embodiment of the invention,computing circuitry 24 compares the output of detector 18 with thestored value and when the signal rises above a threshold value, tonerconcentrate is added to the system.

It will be understood that using the inverse of the preferredembodiment, i.e., having a wide angle detector close to the flow ofliquid, the sensitivity to colored particles is reduced and that toblack is increased. By placing the detector close to the flow, thesensitivity to black can be made many times as high as that to yellow.

In one embodiment of the invention two measurements are made. In a firststep a determination of the black concentration is made as described inthe previous paragraph. In a second step a narrow angle measurement ismade and a correction to the results is made for black contamination.Such correction will be based on prior measurements of the effects ofvarious levels of black contamination on the narrow angle measurements.

The wide angle measurement can be made with a separate source of lightor in a preferred embodiment of the invention shown in FIG. 2, a seconddetector 26 viewing the dispersion is placed near lens 16. If thisdetector is small it will block very little light from detector 18 andhave a wide viewing angle of the dispersion. For this configuration, thedistance between lenses 12 and 16 should be maximized to reduce thesensitivity to yellow.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined only by the claims which follow:

We claim:
 1. A method for detecting the concentration of colored tonerparticles in a dispersion undesirably contaminated by black tonerparticles, the method comprising the steps of:providing a quantity ofthe dispersion of colored toner particles optionally contaminated withblack toner particles; illuminating the dispersion; detecting an amountof light passed through the dispersion with a viewing angle of less than±10 degrees; and determining the concentration of the colored tonerparticles utilizing the detected amount of light.
 2. A method accordingto claim I and including, as a preparatory step, the step of determiningthe sensitivities of the detected light to known concentrations ofcolored toner particles.
 3. A method according claim 2 wherein theviewing angle is less than about ±5 degrees.
 4. A method according toclaim 3 and including the step of correcting for the black tonerparticle contamination in the dispersion.
 5. A method according to claim4 wherein the method includes the steps of:further detecting an amountof light passed through the dispersion with a high viewing angle; anddetermining the concentration of the black toner particles utilizing thedetected amount of light.
 6. A method according to claim 2 and includingthe step of correcting for the black toner particle contamination in thedispersion.
 7. A method according to claim 6 wherein the method includesthe steps of:further detecting an amount of light passed through thedispersion with a high viewing angle; and determining the concentrationof the black toner particles utilizing the detected amount of light. 8.A method according claim 1 wherein the viewing angle is less than about±5 degrees.
 9. A method according to claim 8 and including the step ofcorrecting for the black toner particle contamination in the dispersion.10. A method according to claim 9 wherein the method includes the stepsof:further detecting an amount of light passed through the dispersionwith a high viewing angle; and determining the concentration of theblack toner particles utilizing the detected amount of light.
 11. Amethod according to claim 1 and including the step of correcting for theblack toner particle contamination in the dispersion.
 12. A methodaccording to claim 11 wherein the method includes the steps of:furtherdetecting an amount of light passed through the dispersion with a highviewing angle; and determining the concentration of the black tonerparticles utilizing the detected amount of light.
 13. Apparatus fordetecting concentrations of colored toner particles in the presence ofblack or other absorbing toner particles comprising:a light source; alight detector having a field of view of less than about ±10 degrees;means for supplying a dispersion of colored toner particles optionallycontaining undesirable contamination by black toner particles betweenthe light source and the detector; and computing circuitry operative fordetermining the concentration of the colored toner particles utilizingoutput from the light detector.
 14. Apparatus according to claim 13wherein the light source comprises:a laser diode; and a collimator. 15.Apparatus according to claim 14 wherein the light detector comprises afocusing lens.
 16. Apparatus according to claim 15 wherein the field ofview is limited by the size of the detector.
 17. Apparatus according toclaim 15 wherein the field of view is limited by the size of the lens.18. Apparatus according to claim 14 wherein the light detector comprisesan iris which limits the field of view.
 19. Apparatus according to claim18 wherein the viewing angle is less than about ±5 degrees. 20.Apparatus according to claim 18 and including:a second light detectorhaving a large field of view; and computing circuitry operative fordetermining the concentration of the black toner particles utilizingoutput from the second light detector.
 21. Apparatus according to anyclaim 14 and including means for determining the sensitivities of thedetected light to known concentrations of colored toner particles. 22.Apparatus according to claim 21 and including means for determining theconcentration of black toner particles contaminating the dispersion. 23.Apparatus according to claim 14 wherein the viewing angle is less thanabout ±5 degrees.
 24. Apparatus according to claim 14 and includingmeans for determining the concentration of black toner particlescontaminating the dispersion.
 25. Apparatus according to claim 14 andincluding:a second light detector having a large field of view; andcomputing circuitry operative for determining the concentration of theblack toner particles utilizing output from the second light detector.26. Apparatus according to any claim 14 and including means fordetermining the sensitivities of the detected light to knownconcentrations of colored toner particles.
 27. Apparatus according toclaim 13 wherein the light detector comprises a focusing lens. 28.Apparatus according to claim 27 wherein the field of view is limited bythe size of the detector.
 29. Apparatus according to claim 27 whereinthe field of view is limited by the size of the lens.
 30. Apparatusaccording to claim 13 wherein the light detector comprises an iris whichlimits the field of view.
 31. Apparatus according to claim 30 whereinthe viewing angle is less than about ±5 degrees.
 32. Apparatus accordingto claim 30 and including:a second light detector having a large fieldof view; and computing circuitry operative for determining theconcentration of the black toner particles utilizing output from thesecond light detector.
 33. Apparatus according to claim 13 and includingmeans for determining the sensitivities of the detected light to knownconcentrations of colored toner particles.
 34. Apparatus according toclaim 33 and including means for determining the concentration of blacktoner particles contaminating the dispersion.
 35. Apparatus according toclaim 13 wherein the viewing angle is less than about ±5 degrees. 36.Apparatus according to claim 13 and including means for determining theconcentration of black toner particles contaminating the dispersion. 37.Apparatus according to claim 13 and including:a second light detectorhaving a large field of view; and computing circuitry operative fordetermining the concentration of the black toner particles utilizingoutput from the second light detector.